1. Field of the Invention
The present invention relates to an all-pass reverberator and more particularly to a reverberator having an MOS delay line.
2. Description of the Prior Art
The present invention starts from an article by M. R. Schroeder entitled "Natural Sounding Artificial Reverberation" and published in "Journal of the Audio Engineering Society," 10/3, (July 1962), pp. 219 to 223. That article deals with a basic circuit for an all-pass reverberator with a delay line fed back via a feedback amplifier and a first adding circuit at the input of the basic circuit and connected in series with an output amplifier between the output of the first adding circuit and a first input of a second adding circuit connected to the output of the basic circuit the second input of which second adding circuit is connected to the output of a signal amplifier whose input is connected to the input of the first adding circuit.
In the aforementioned article a delay line is required which has neither any gain nor attenuation nor frequency response, i.e. with g.sub.d = 1, whereby the attenuation of the delay line will be designated in the following. Another teaching of that article is that three amplifiers or attenuators with definite gains or attenuation factors are necessary to achieve all-pass characteristics. By contrast, the present invention relates to a basic circuit for an all-pass reverberator which uses as the delay line an MOS delay line as known from, e.g., the journal "Elektor" (January 1973), pp. 112 to 117. Such use presents special problems regarding the design and alignment of the amplifiers, whose gain must be frequency-dependent.
In practice, an MOS delay line has a frequency-dependent attenuation given by ##EQU1## WHERE F = SIGNAL FREQUENCY
f.sub.c = clock frequency PA1 n.sub.d = number of stages (determining the time delay .tau.) PA1 .delta..sub.m = maximum stage attenuation at f = f.sub.c /2. PA1 1. Use must be made of a delay line employing signal regeneration, whereby the exponential factor in (1) is made to be equal to 1 to a good approximation. For this purpose, well-proven signal regeneration circuits are available. PA1 2. g.sub.o must be made to be equal to 1. PA1 g.sub.x is the gain of the feed back amplifier, PA1 g.sub.y is the gain of the output amplifier, and PA1 g.sub.z is the gain of the signal amplifier.
For a large n.sub.d .delta..sub.m, therefore, appreciable deviations from the all-pass behavior are to be expected. To fulfill Schroeder's requirement g.sub.d = 1, two measures must be taken: